Liquid Crystalline PolymerâCo Nanorod Hybrids: Structural Analysis

Article pubs.acs.org/JPCB

Liquid Crystalline Polymer−Co Nanorod Hybrids: Structural Analysis and Response to a Magnetic Field Ophélie Riou,†,‡ Barbara Lonetti,†,* Patrick Davidson,§ Reasmey P. Tan,‡ Benoit Cormary,‡ Anne-Françoise Mingotaud,† E. Di Cola,⊥ Marc Respaud,‡ Bruno Chaudret,‡ Katerina Soulantica,‡ and Monique Mauzac† †

Laboratoire des Interactions Moléculaires et Réactivité Chimique et Photochimique, Université de Toulouse, UPS/CNRS, 118 route de Narbonne, F-31062 Toulouse Cedex 9, France ‡ Laboratoire de Physique et Chimie de Nano-Objets, Université de Toulouse, INSA, UPS, LPCNO, CNRS, 135 avenue de Rangueil, F-31077 Toulouse Cedex 9, France § Laboratoire de Physique des Solides, UMR 8502 CNRS, Université Paris-Sud, Batiment 510, 91405 Orsay Cedex, France ⊥ European Synchrotron Radiation Facility−ESRF, F-38043 Grenoble Cedex, France S Supporting Information *

ABSTRACT: This work deals with the structural analysis of side-chain liquid crystalline polysiloxanes, doped with magnetic cobalt nanorods, and their orientational properties under a magnetic field. These new materials exhibit the original combination of orientational behavior and ferromagnetic properties at room temperature. Here we show that, within the liquid crystal polymer matrix, the cobalt nanorods self-assemble in bundles made of nanorod rows packed in a 2dimensional hexagonal lattice. This structure accounts for the magnetic properties of the composites. The magnetic and orientational properties are discussed with respect to the nature of the polymer matrix.

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INTRODUCTION

orientational order of the liquid crystalline units integrated in the polymer matrix and the rubber elasticity of the polymer network. Hence, macroscopically oriented liquid crystalline elastomers exhibit large and reversible deformations when they undergo a liquid crystalline to isotropic phase transition. Temperature is the most usually employed stimulus for actuating liquid crystalline elastomers but light or electric fields have been also considered.24,25 Dispersing magnetic particles within these elastomers could provide shape-changing materials through application of a magnetic field during the isotropic/ liquid crystal phase transition. Moreover, the use of magnetic particles should enhance the field-induced orientational response of liquid crystals, which was first proposed by Brochard and de Gennes in the 1970s26 and then experimentally confirmed some years later.27−31 It has been shown that doped nematic liquid crystals could be oriented by a magnetic field intensity 103 times smaller than the usual.28,31 These studies concluded that the two key parameters in order to achieve a strong orientational coupling between magnetic nanoparticles and low molecular weight liquid crystals are (i) the efficiency of anchoring of the liquid crystal molecules

Organic−inorganic hybrid materials are increasingly studied in an effort to produce stimuli-responsive smart systems. In particular, composites obtained by mixing magnetic nanoparticles (most frequently iron particles of nanometer or micrometer sizes) with deformable polymer matrices open new perspectives in the field of information storage,1,2 biomedicine,3 and actuation.4−6 Their mechanical properties, such as, for example, shape change (using an electromagnetically induced heating on doped shape-memory polyurethanes or polyacrylates5−7) and tunable elastic modulus (doped polysiloxanes cross-linked in the presence of a magnetic field6,8), offer an attractive alternative to the older rigid magnetic networks usually characterized by low flexibility and small shape changes.9−11 The ability of these materials to respond to external stimuli is due to the collective properties of particles organized in the medium. In order to obtain nanostructured materials, different experimental approaches have been used: block copolymers12−14 or lyotropic templates15−17 are the most common strategies employed. Recently liquid crystalline polymers have also been used.18−20 The long-term objective is to obtain “smart” thermotropic elastomers: materials that can display shape-memory properties triggered by the thermal transition of the liquid crystalline domains. 21−23 This phenomenon is due to the strong coupling between the © 2014 American Chemical Society

Received: October 9, 2013 Revised: February 7, 2014 Published: February 19, 2014 3218

dx.doi.org/10.1021/jp410050z | J. Phys. Chem. B 2014, 118, 3218−3225

The Journal of Physical Chemistry B

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Scheme 1. Structure of the Liquid Crystalline Polymers Used

on nanoparticles, and (ii) the torque induced by the applied magnetic field on the medium. Thus, efficiency of anchoring is strongly enhanced through the use of rod-like shaped nanoparticles. Furthermore, the employment of strong magnetic materials like metallic Fe, Co, etc. instead of the weak magnetic iron oxide materials guarantees a larger torque with smaller, nanometer-size, nanoparticles. In this context, we have optimized the elaboration of a new material where cobalt nanorods are dispersed in a liquid crystalline polymer.32,33 We observed that the obtained hybrids exhibit simultaneously mesomorphous and ferromagnetic properties: the liquid crystal order is maintained in the presence of the nanorods and the magnetic properties are improved compared to the properties of the same nanorods in an inert matrix.33 In the present study we report the results of new SAXS (small angle X-ray scattering) and SQuID (superconducting quantum interference device) experiments which now allow us to gain a deeper insight both into the orientational order of the liquid crystal moieties within the hybrids compared to the initial polymers, and into the orientational response of the liquid crystalline moieties to an applied magnetic field. Moreover, through the analysis of the orientational response of the liquid crystal moieties to an applied magnetic field for two different polymers, we have been able to assess the effect of the presence on the polymer chain of carboxylic lateral groups that interact with Co nanorods.

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organic ligands (20% by weight). We have chosen to work with these purified nanorods in order to limit the content of organic ligands. Indeed, liquid crystalline phases are very sensitive to the presence of impurities and are easily destabilized by even low concentrations of organic molecules. Nevertheless, the presence of a small amount (20% by weight) of ligands impedes strong aggregation and facilitates the homogeneous dispersion of the nanorods in the liquid crystal matrix, without altering it. Because of the sensitivity of cobalt toward oxidation, all manipulations were performed in a glovebox. The liquid crystalline polymers (LCPs) used are statistical copolymers composed of a flexible polysiloxane chain and one (P41) or two (P41BoBa) types of mesogenic moieties: the 4′(3-butenyloxy)phenyl-4-(methoxy)benzoate and 4-(3butenyloxy)benzoic acid (Scheme 1).36,37 The latter polymer was chosen in order to favor the interaction between the polymer and the Co nanoparticles. Indeed, it is well-known that carboxylic groups strongly bind to Co nanoparticles.38,39 The magnetic liquid crystalline polymer (MLCP) hybrids were obtained by mixing preformed purified cobalt nanorods (0.2 wt % to 1.2 wt %) with the liquid crystalline polymer dissolved in toluene; the solutions were sonicated during 15 min at room temperature and the toluene was slowly evaporated under vacuum while the solutions were mechanically stirred overnight. No sedimentation was observed for at least one year and the materials appeared homogeneous at the macroscopic level. On the other hand, the nanorod dispersion was relatively inhomogeneous at the microscopic level (Figure 1), as already observed in TEM images of ultrathin polymer films with 4 wt % nanorods.33 Methods. Differential Scanning Calorimetry. Differential Scanning Calorimetry (DSC) measurements were performed with a DSC1 Star System (Mettler Toledo). The transition temperatures were determined from the position of the top of the DSC peaks at a cooling rate of 10°/min and the glass transition temperature from the inflection point of the heating cycle at a rate of 10°/min. The values of the transition temperatures (reported in Table 1) are the averages of three measurements on different samples from the same synthesis. X-ray Scattering. Samples were studied with four different set-ups in order to probe their structure at different length scales. The nonoriented samples have been measured at the European Synchrotron Radiation Facility, ID2 beamline, and the spectra have been recorded using a wavelength λ = 0.1 nm and a sample to detector distance of 1 m.

EXPERIMENTAL SECTION

Materials. Toluene was purchased from Fischer Chemical (purity 99%), hexadecylamine (HDA, 99%) from Fluka (purity 99%), lauric acid (LA, 99.5%) from Acros-Organics, poly(methylhydrosiloxane) (PHMS) from Aldrich, and cisdichlorobis(diethylsulfide) platinum II, (SEt2)2PtCl2, from Aldrich. The precursor Co[N(SiMe3)2]2(thf) was synthesized according to a published procedure.34 The cobalt nanorods were synthesized from Co[N(SiMe3)2]2(thf) by slightly modifying a published protocol.35 More precisely the Co precursor (0.84 equiv) was rapidly added at room temperature to an anisole solution containing HDA (1.4 equiv) and LA (1 equiv). The resulting solution (V = 20 mL) was put under a pressure of 3 bar of hydrogen and then heated at 150 °C during 24 h. Monocrystalline metallic nanorods of 66 ± 8 nm length and 5.5 ± 0.5 nm diameter were obtained (Figure 1S, Supporting Information). As verified by elemental analysis, the dry powder obtained after washing the nanorods repeatedly with toluene contains a small quantity of 3219



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phase, at room temperature, in the presence of magnetic fields of 0.0625, 0.125, 0.25, 0.5, 1, 1.4, and 1.7 T, successively. We checked that the high-temperature annealing step in zero-field, performed before applying each value of field intensity, effectively erased any previous alignment. The organization and alignment of the Co nanorods were examined with another setup equipped with a Cu rotatinganode generator.43 A parallel monochromatic X-ray beam was obtained from an Osmic multilayered optics. The detection was achieved either with an imaging plate or with a Princeton CCD camera. The sample to detection distance was either 1m or 50 mm, in small-angle (SAXS) or wide-angle (WAXS) scattering configurations, respectively. Finally, several samples were studied at the Swing SAXS beamline of the SOLEIL synchrotron facility (Gif-sur-Yvette, France). This experimental station allows recording signals scattered at q values as low as q = 2 × 10−2 nm−1 (where q is the scattering vector modulus, q = (4π sin θ)/λ, where 2θ is the scattering angle and λ = 0.1033 nm is the wavelength). The sample-detection distance was either 1.07 or 6.37 m and typical exposure times were 10 ms. Magnetic Measurements. Superconducting Quantum Interference Device, SQuID (MPMS 5 QUANTUM DESIGN) was used to characterize the magnetic properties of the polymer composites. The sample was heated up to the isotropic phase and then, in the presence of a 5 T magnetic field, cooled down to 300 K where the magnetization was recorded as a function of the applied field. For the measurements, the samples were introduced in gelatin capsules in order to prevent any oxidation, kept in a sealed Schlenk, and rapidly transferred into the measurement cell.

Figure 1. TEM image of a MLCP showing the bundles that are present in the sample. Bundles or even isolated nanorods perpendicularly oriented with respect to the TEM grid are surrounded by a blue circle.

The orientation of the mesogenic side groups and the smectic order was studied using an X-ray scattering setup specifically designed for thermotropic liquid crystals and already described.40 The Cu Kα radiation emitted by a fixed X-ray tube was selected and focused by a graphite doubly curved monochromator to obtain a point-like beam. The sample, held in a vertical, flame-sealed, cylindrical Lindemann glass capillary (diameter 1.5 mm, Mark-Rohrchen, Germany), was introduced in an oven, with ±1 K accuracy, itself placed between the poles of an electromagnet. The magnetic field could be varied between 0 and 1.7 T. The camera was evacuated in order to avoid air scattering and measure very low scattering signals. Scattered X-rays were detected on a cylindrical imaging plate set at 60 mm from the sample. Exposure times were typically a few hours. Using a classical procedure, the nematic order parameter, S, of the mesogenic side groups can be obtained from an azimuthal scan of the X-ray intensity scattered in the wide-angle diffuse ring.41,42 In order to examine the influence of Co nanorod doping on the orientational response of the hybrid materials submitted to a magnetic field, samples were aligned under different field intensities. The following protocol was used: after annealing for 30 min in the isotropic phase (∼135 °C), in zero field, samples were slowly cooled down (rate of 0.2 °C/min) to the smectic

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RESULTS AND DISCUSSION The liquid crystalline character of the different materials was analyzed by DSC and X-ray scattering experiments. Note that observations by polarized-light microscopy were very difficult because of the strong absorption of light by the Co nanorods, even at rather low concentrations, so that proper examination of textures could not be achieved. The initial polymers exhibited both a nematic and a smectic A phase.37 The same mesophases were observed in the MLCPs (see the following X-ray scattering characterization), showing that the polymorphism was preserved in the presence of nanorods. The transition temperatures of the initial and doped polymers are listed in Table 1.

Table 1. Transition Temperatures of the Doped Liquid Crystalline Polymers and the Liquid Crystalline Polymers As Obtained from Differential Scanning Calorimetry polymer matrix

wt % NR

P41 P41 P41 P41 P41 P41Boba P41Boba

0 0.2 0.6 0.8 1.2 0 0.8

Tg (°C) 4.7 8.5 7.3 7.2 7.4 8.8 9.1

± ± ± ± ± ± ±

1.0 1.0 1.0 1.0 1.0 1.0 1.0

TI/N (°C) 106.3 107.9 108.1 108.4 108.8 97.4 99.2

± ± ± ± ± ± ±

0.2 0.6 0.3 0.2 0.4 0.5 0.6

ΔHI/N (J/g)

TN/SmA (°C)

ΔHN/SmA (J/g)

± ± ± ± ± ± ±

74a n.d. n.d. n.d. n.d. 90.2 ± 1.0b 88.0 ± 2.0b

n.d. n.d. n.d. n.d. n.d. 0.50 ± 0.10b 0.29 ± 0.10b

1.20 1.20 1.20 1.20 1.20 1.19 1.13

0.10 0.10 0.10 0.10 0.10 0.10 0.10

TI/N: isotropic/nematic transition temperature. Tg: glass transition temperature. ΔHI/N isotropic/nematic transition enthalpy. ΔHN/SmA nematic/smectic A transition enthalpy. aValue from ref 37. bThe smectic to nematic transion temperature and the enthalpy change have been determined by a fit of the data with two Gaussian curves. 3220



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The nematic to smectic transition temperature could not be determined by DSC for the P41 polymer due to its too low enthalpy change. In the case of P41Boba the presence of the acid moiety stabilizes the smectic phase and the smectic/nematic transition could be detected: its peak appears as a shoulder on the side of the nematic/isotropic one. When nanorods were added, the temperature of the nematic to smectic phase transition remained constant but the transition enthalpy decreased: the nanorod doping slightly destabilizes the smectic phase, as also observed from X-ray data. In all hybrids, a slight increase of the clearing temperature occurred upon nanorod (Figure 2) doping but the transition Figure 3. X-ray scattering patterns of magnetically aligned samples of (a) P41, (b) P41 + 0.6 wt % Co nanorods, (c) P41Boba, and (d) P41 Boba +0.8 wt % Co nanorods, recorded at room temperature. The vertical black arrow in part d shows the direction of the magnetic field for all patterns. In part a, the solid black arrow points to the wide-angle diffuse ring, the dotted black arrow points to the smectic reflection close to the beamstop (white disk), and the dashed black arrow points to faint diffuse streaks due to the form factor of the mesogenic side groups.

Table 2. Lattice Parameter [Å] of the Smectic Phase for the Two Polymers, P41 and P41Boba, at Different Nanorod Concentrations

Figure 2. DSC traces of the MLCP obtained from P41, the arrow indicates the increasing nanorod content from 0 to 1.2 wt %.

wt % NR

P41

P41Boba

0 0.8 3.2

34.4 35.0 34.5

33.4 34.5 35.1

When both polymers were mixed with the nanorods, the lattice parameter of the smectic phase was not altered by the presence of nanorods for the P41 polymer but it slightly increased, upon doping, for P41Boba. The organization of the Co nanorods within the hybrid materials, aligned in a 1.7 T magnetic field, was investigated with the SAXS Laboratory setup (Figure 4). Strong scattering spots are observed in the direction perpendicular to the magnetic field and correspond to the intersection with the

enthalpy was not affected. This phenomenon has already been observed with similar polymers doped with various proportions of Co particles.32 This effect does not seem to be due to the ferromagnetic properties of the nanorods since experiments performed on oxidized nonmagnetic samples gave the same results. A recent example in the literature showed that fluxional LC ligands characterized by a certain degree of freedom can preferentially organize around the nanoparticles, thus improving the LC properties.44 The organization of the LCP matrix of the initial polymers, P41 and P41 BoBa, and of two MLCPs was investigated by recording their X-ray scattering patterns at room temperature in the SmA phase. All these patterns are very similar as shown in Figure 3. As expected, they show: (i) at wide scattering angles the anisotropic diffuse halo due to the mesogenic cores aligned along the magnetic-field direction, and (ii) the smectic reflections at small angles. Faint scattering streaks perpendicular to the magnetic field direction are due to the form factor of the mesogenic cores.45 Compared to other types of side-chain liquid crystalline polymers, the smectic reflections are rather weak, which shows that the smectic order parameter is small.45 Moreover, close inspection of the patterns (and additional scattering experiments on nonoriented samples, data not shown) reveals that the intensities of the smectic reflections slightly decrease upon nanorod doping, which would suggest a destabilization of the smectic phase. The values of the smectic periods are given in Table 2. These values are in accordance with those found in the literature: the length of the mesogenic group in its more extended conformation is 24.1 Å. This structure corresponds to a partially bilayered structure.37

Figure 4. (a) SAXS pattern of a magnetically aligned sample of P41Boba + 0.8 wt % Co nanorods. The direction of the magnetic field B is shown by the solid arrow at the bottom. The solid arrows at the top show the (110) and (100) reflections due to the hexagonal order of Co nanorods in bundles. The dotted arrow shows SAXS signals arising from the longitudinal order of the nanorods in rows. The dashed line was used to plot the intensity scattered by the nanorods in the plane perpendicular to the magnetic field, shown in part b. 3221



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detection plane of diffuse rings, at q = 0.61 nm−1, arising from interferences between Co nanorods. A plot of the scattered intensity (Figure 4b) reveals the presence of another weak peak at a wave vector, q = 1.06 nm−1, larger by a factor 31/2 than that of the first one. These data show that the Co nanorods form bundles within the hybrid materials, with a hexagonal symmetry in the plane perpendicular to the bundle axis. The hexagonal lattice parameter, a = 10.3 nm, is larger than the average Co nanorod diameter (5.5 nm), which confirms that the Co nanorods are actually coated by P41Boba macromolecules. The formation of Co nanorod bundles is hardly surprising since a rough estimate of ∼60 kBT can be obtained for the magnetic dipolar attractive interaction between these ferromagnetic objects, ∼ 10 nm apart. Therefore, although the hybrid materials seemed fairly homogeneous at the macroscopic level, the Co nanorods gathered in bundles at the microscopic scale. These bundles could also be observed in TEM images (Figure 1 and Figure 2S). This inhomogeneity at the microscopic level is inherent to the system and difficult to avoid;32 nevertheless we checked that it does not affect the reproducibility and the quality of the mesomorphous and magnetic properties, as different synthesis batches led to the same results. All the SAXS patterns of the hybrid materials investigated in this study displayed such diffuse rings, proving that Co nanorods always formed bundles with a hexagonal lattice parameter that varied roughly between 8 and 14 nm. This nanorod organization is somewhat reminiscent of the formation of carbon-nanotube bundles.46 The narrow (azimuthal) angular extension of the (100) scattering peaks arising from the hexagonal order of the Co nanorods within the bundles shows that they are very well aligned by the magnetic field. Their nematic order parameter, SCoNR, can be derived from the angular profiles of these (100) peaks in exactly the same way as that of the mesogenic cores. The value thus obtained, SCoNR ∼ 0.85, is very large and illustrates the high orientational response of the Co nanorods to the magnetic field in the hybrid materials, in good agreement with the magnetic measurements (see below). We observed that this very good alignment was already obtained at field intensities as low as ∼100 mT. Close inspection of Figure 4 shows that there are additional SAXS signals at very small angles along the magnetic-field direction. These signals can be more conveniently observed on SAXS patterns recorded at the Swing beamline of the SOLEIL synchrotron (Figure 5). In addition to the sharp reflection due to the smectic order of the mesogenic side groups and to the diffuse spots due to the hexagonal order of the Co nanorods within the bundles, this SAXS pattern indeed shows an additional diffuse spot at small angles. This experiment and others (data not shown), at larger sample−detector distance, confirmed the existence of equidistant (001), (002), and (003) reflections that correspond to a periodicity of 80 ± 5 nm along the bundle axis. These scattering features are very likely due to rows of Co nanorods within the bundles, as expected for magnetic dipoles. The period within these rows agrees rather well with the average length, 66 ± 8 nm, of the nanorods if the thickness of the polymer coating is also considered. Therefore, it appears that Co nanorods, due to their magnetic dipoles, selfassemble into rows that further pack in a 2-dimensional hexagonal lattice within bundles. This conclusion applies to all hybrid materials that we studied. In order to examine the influence of Co nanorod doping on the orientational response of these materials to a magnetic field,

Figure 5. (a) SAXS pattern, recorded at the Swing beamline, of the P41 Boba +0.8 wt % Co nanorods hybrid material. The solid white arrow points to the (001) reflection due to the organization of the Co nanorods in rows within the bundles whereas the dashed arrows point to the (100) and (110) reflections of the hexagonal 2-dimensional lattice. The dotted white arrow points to the smectic reflection from the mesogenic side groups. (b) Schematic representation of the nanorod bundles in the MLCP.

we measured the nematic order parameter of the mesogenic side groups in samples aligned at different field intensities, using the protocol described in the Experimental Section (Figure 6). Although magnetic measurements and SAXS studies showed that Co nanorods aligned in field intensities as low as 100 mT, doping P41 with Co nanorods (Figure 6a) did not improve much the orientational response of the polymer material. Indeed, both in the presence and in the absence of nanorods, the P41 polymer was quite easily oriented under magnetic field and already at 62.5 mT, the liquid crystal groups are slightly aligned. Moreover, the maximum order parameter S = 0.67 ± 0.05 was achieved at 0.5 T in both cases. Finally, no influence of the concentration of Co nanorods was observed within our experimental accuracy. In contrast, doping P41Boba with Co nanorods slightly improved the orientational response as the doped material showed a better alignment when field intensities in the range 0.25−0.5 T were applied (Figure 6b). Indeed, P41Boba was harder to align than P41, which makes Co nanorod doping more effective. Under field intensities larger than about 0.5 T, all materials were very well aligned and doping was not necessary. The fact that Co nanorod doping was more effective for materials prepared from P41Boba than from P41 may be due to the stronger chemical interaction between the former initial polymer and the Co nanorods. The difference between the two polymers consists in the presence of 5%mol carboxylic acid groups, in the case of P41Boba, that are known to be strong ligands for Co.38,39 Unfortunately, for this kind of polymer, the proportion of carboxylic groups cannot be increased any further since it induces some cross-linking reactions in the polymer matrix. The magnetic properties of these composites were also investigated. The main elements of the hysteresis curves relative to the different samples are reported in Table 3. Figure 7 shows typical hysteresis loops of the composites between −50kOe and 50kOe obtained at 300 K after field cooling the samples from the isotropic phase. All the composites are ferromagnetic at room temperature. The alignment of the Co nanorods along the field inside the matrix is inferred from the magnetization curves as a function of the applied magnetic field. In fact, the hysteresis loops were almost square shaped with a high remanent magnetization in 3222



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Figure 6. Variations of the order parameter of the mesogenic side groups as a function of the magnetic field intensity applied to the sample upon cooling from 125 to 25 °C at a rate of 0.2 °C/min: (a) P41 (black squares), P41 + 0.2 wt % Co nanorods (red circles), and P41 + 0.6 wt % Co nanorods (blue triangles); (b) P41Boba (black squares), P41Boba + 0.8 wt % Co nanorods (red circles).

theoretical ones (∼1.4 T). In the case of P41Boba (HC ∼ 0.8 T), these values are still below the theoretical ones, but they are enhanced compared to the highly packed Co nanorods assemblies. To the best of our knowledge, they are even the largest ever measured on elemental 3d metallic ferromagnetic nano-objects.47,48 By means of X-ray scattering, we have shown that Co nanorods self-assemble into rows that further organize in a hexagonal lattice, giving rise to highly ordered structures within the hybrid materials. The better alignment is achieved in the presence of carboxylic groups in the polymer matrix, which leads to a more intimate mixture of the two organic/inorganic components.

Table 3. Coercive Field, Hc, and Remanent Magnetization to Spontaneous Magnetization Ratio, Mr/Ms at 300 K for the Neat Nanorods and Different Composites polymer matrix

wt % NR

Hc [T]

Mr/Ms

neat nanorods P41 P41 P41 P41Boba P41Boba

80 1.2 0.6 0.2 0.8 2.5

0.56 0.62 0.76 0.55 0.87 0.82

0.88 0.82 0.78 0.67 0.96 0.94

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CONCLUSIONS

In this work, we have shown that mesomorphic side-group polymers can be doped with Co nanorods without destabilizing the liquid crystalline phases while preserving the magnetic properties of the nanoparticles. Two different polymers have been tested, and it has been shown that both the magnetic and orientational properties of the final material are improved when a strong interaction between nanorods and side groups is achieved. Moreover, due to strong dipolar attractive interactions, Co nanorods form rows that self-assemble into wellorganized bundles with hexagonal order. Such bundling is probably detrimental to the intended improvement of the fieldinduced orientational order of the mesogenic groups, but it should be very hard to avoid if strongly magnetic particles are to be used. We have also established a relationship between the original microscopic structure of the material and its excellent magnetic properties. The nanorod concentration used in these experiments was kept quite low on purpose, but one might expect that Co nanorods assembly could occur over a longer range if nanoparticle concentrations are increased. The structure of the samples was studied at different length scales, providing a thorough insight of these new polymer hybrids. As previously described,32 cross-linked composites can be obtained from these doped polymers, leading then to ferromagnetic liquid crystalline elastomers that are promising materials for applications such as actuators.

Figure 7. Hysteresis loops of the composites containing P41Boba + 0.8 wt % Co nanorods (solid line), P41 + 1.2 wt % Co nanorods (circles and solid line), and 80 wt % Co nanorods in tetracosane (dashed point line).

the range between 67−82% and 94−96% of the spontaneous magnetization, and a high coercive field (HC) ∼ 0.6−0.8 T, for P41 and P41Boba, respectively. Comparable values have already been observed in highly packed Co nanorods aligned inside a simple wax matrix.47 In that work, the authors demonstrated that the high coercive field resulted from the contribution of both high magneto-crystalline (due to hcp structure) and shape anisotropy, much stronger than dipolar interaction fields, in the framework of coherent reversal of the magnetization. However, an average misalignment of the nanorods of 10−20° with respect to the applied magnetic field led to experimental values of HC (6.5kOe) lower than the 3223



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ASSOCIATED CONTENT

S Supporting Information *

TEM image of the purified nanorods, Figure 1S, and other typical TEM images of the nanocomposite, Figure 2S. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(B.L.) E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank ANR Agency (MaCriLiMa JC&JC2010), the EU (FEDER-35477 “Nano-objets pour la biotechnologie”), and the European Commission FP7 NAMDIATREAM project (EU NMP4-LA-2010-246479) for financial support and A. Mari and L. Rechignat for SQuID measurements. We thank Florian Meneau for help during the SAXS experiments at the Swing beamline of the SOLEIL synchrotron facility. We thank M. Sztucki for help for the SAXS experiments at ID2 beamline of ESRF.

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REFERENCES

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Liquid Crystalline PolymerâCo Nanorod Hybrids: Structural Analysis

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